Although batteries have long been used, they have recently been recognized as one of the extremely important base components, together with semiconductors and liquid crystal display elements, in the information age. Particularly in the cases of portable telephones and notebook type personal computers, the demands for improved battery performance and reduced battery weight are strong, and lithium ion batteries are attracting considerable attention as one type of battery capable of meeting these demands. Lithium ion batteries offer higher energy densities and more rapid charging times than other batteries such as nickel-cadmium batteries, and offer considerable promise for the future.
In electrochemical elements such as primary batteries, secondary batteries and capacitors, liquids have traditionally been used as the electrolyte, but liquid electrolytes are prone to leakage and do not provide ideal long-term reliability. In contrast, solid electrolytes do not suffer from these drawbacks, and the application of such solid electrolytes to a variety of electrochemical elements not only simplifies the production of the element, but also enables reductions in the size and weight of the element, and because of the lack of any leakage problems, enables the provision of a highly reliable element.
Accordingly, in the field of lithium ion battery research, the research and development of new solid electrolytes, and in particular the research and development of solid polymer electrolytes that are lightweight, flexible and readily processed, is being actively pursued.
Almost all polymer compounds are insulators, but since the announcement that certain polymer materials such as polyethylene oxide (PEO) are capable of forming a crystalline complex with electrolyte salts such as lithium salts, resulting in a high ionic conductivity, much attention has been focused on research into solid polymer electrolytes that use PEO or other polyalkylene oxides, or polyethyleneimines or polyphosphazenes that contain similar ion dissociation groups as the electrolyte matrix. There have been many publications of research relating to solid polymer electrolytes in which a polyalkylene oxide represents one component of the matrix, and the ionic conductivity at room temperature has now reached 10−4 to 10−6 S/cm. However, in order to achieve a high ionic conductivity, the polyalkylene oxide content within the matrix must be increased, but unfortunately this leads to a marked deterioration in the strength and heat resistance of the electrolyte film, meaning producing a practical solid electrolyte has been difficult. Moreover, the ionic conductivity also decreases dramatically if the temperature falls to 0° C. or lower (see Japanese Unexamined Patent Application, First Publication No. Hei 5-120912, and J. Appl. Electrochem., No. 5, 63 to 69 (1975)).
A solid polymer electrolyte in which the matrix substrate comprises an ABA type triblock copolymer produced by copolymerizing methoxypolyethylene glycol monomethacrylate (A) and styrene (B) via a living anionic polymerization has been proposed as a suitable solid polymer electrolyte (see Makromol. Chem., 190, 1069 to 1078 (1989)).
However, the homopolymer of the methoxypolyethylene glycol monomethacrylate of the component (A) is a liquid at room temperature, even at very high molecular weights, meaning that in order to ensure that the A-B-A copolymer forms a solid electrolyte matrix substrate, the quantity of the component (A) must be restricted. However, this means that the shape and size of the PEO domain, which functions as the diffusion and transport space for the lithium ions, is also restricted, and as a result, the ionic conductivity at 40° C. was a somewhat unsatisfactory 10−6 S/cm.